Polarization enhanced low frequency NQR detection
J. Luznik, J. Pirnat, Z. Trontelj, J. Seliger and T. Apih
Institute of Mathematics, Physics and Mechanics, Faculty of Mathematics and Physics and Institute J. Stefan, University of Ljubljana, Slovenia
Introduction
Almost all explosives and drugs contain nitrogen. 14N NQR spectra and their resonance frequencies depend on the molecular structure and are characteristic of each of these substances, providing a kind of their
»fingerprint« and so enabling their detection and identification. The problem is that the 14N NQR spectral lines often lie at low frequencies where the NQR signals are usually very weak and not easy to detect. A known method to improve the signal is polarization enhancement.
Initially protons are polarized in a strong magnetic field so that the proton NMR levels are split considerably more than the nitrogen NQR levels. Due to the Boltzman distribution factor the occupation difference between the high and low NMR levels of protons is much higher than the occupation difference between 14N NQR levels. During the adiabatic demagnetization of the measured sample level crossing between proton NMR and 14N NQR occurs, causing an energy flow from the »hot«
nitrogen quadrupolar system to the »cold« proton NMR system which is manifested in the »cooling« of the nitrogen quadrupolar system.
Applying standard pulse NQR detection techniques immediately after removing the magnetic field, the signal can be improved according to the ratio of the proton NMR to 14N NQR frequency. We can choose between two possibilities (I) to switch off the magnetic field or to remove the magnet [1,2] and (ii) to remove the sample [3].
Experimental
The NQR polarization enhancement increases with the strength of the proton polarization magnetic field. In the limited space within a spectrometer we can obtain a very strong and homogeneous magnetic field. For proton polarization and adiabatic demagnetization magnetic field cycling or sample extraction can be used. When detecting or searching for some remote substance it can usually be approached only from one side. Under such circumstances the effective polarizing magnetic field is dissipated in the half space and drops rapidly with the distance so that in practice polarizing fields are limited to a few hundred mT. Instead of sample transfer permanent magnet removal is performed.
0 100 200 300 400 500
0 5 10
d [cm]
B [m T ]
Fig. 3. Measured magnetic field intensity vs. distance from the magnetic pole surface.
A simple mechanical system for manipulation with the NdFeB block shaped permanent magnet was designed (Fig.1). The time needed to remove the magnet is reduced to aproximately 0,4 sec and we confirmed all our previous measurements [2]. An example of a single shot signal in nitrobenzoic acid where the enhancement factor of about 8 was obtained is shown (Fig.4). At the moment we are involved in the polarization enhancement of the TNT signal, but the CH3 group in the TNT molecule seems to play the dominant role in the cross polarization effect, due to its fast relaxation [4].
References
[1] R. Blinc, J. Seliger, D. Arcon, T. Apih, P. Cevc, V. Zagar, Phys. Status Solidi A, 180, 541 (2000)
[2] J. Luznik, J. Pirnat, Z. Trontelj, Solid State Commun., 121 (2002) 653 [3] V. Mikhalsevitch, A. Beliakov, Phys. Stat . Sol., (2005)
[4] D. Kruk, J. Altman, F. Fujara, A. Gadke, M. Nolte, A.f. Privalov, J. Phys.:
Condens. Matter 17 (2005)519
A)
B)
Fig. 4. Polarization enhancement in A) 4-nitrobenzoic acid single shot with polarization, B) an average of 300 shots without, C) an average of 8 signals
in TNT with polarization and D) average of 2000 signals in TNT without
Fig.1
Fig.2
C)
D)
